Pyridazinone derivatives and salts thereof as cardiac stimulants

ABSTRACT

There are provided novel pyridazinone derivatives having the general formula (I): ##STR1## wherein A represents 5- or 6-membered heterocyclic ring having 1-3 nitrogen atoms, which may be substituted by at least one member selected from the group consisting of C 1-5  alkyl, cyano, hydroxyl, C 1-5  alkoxy, amino, C 1-5  alkylamino, C 2-6  dialkylamino, C 2-5  acylamino, carboxyl, C 2-5  alkoxycarbonyl and carbamoyl, and R 1  and R 2  independently represent hydrogen atom or C 1-5  alkyl or R 1  and R 2  may form together C 1-5  alkylene, and salts thereof.

BACKGROUND OF THE INVENTION

This invention relates to a novel pyridazinone derivative or a salt thereof which is useful for a cardiac stimulant.

Various cardiac stimulants, which effect a direct enhancement of the cardiac contraction, have been available in the treatment of cardiac insufficiency. However, these cardiac stimulants may have some defects such as follows: (1) their safety margin is extremely small; (2) they may cause arrhythmia; (3) their cardiac stimulating activity is transient; and (4) they may not be suitable for oral administration.

A primary object of the present invention is to provide a novel cardiac compound having a high and sustained activity.

SUMMARY OF THE INVENTION

In the course of the present inventors' study onto compounds which act as cardiac stimulants with a high and sustained activity, the present invention has been achieved.

The present invention pertains to pyridazinone derivatives having the general formula (I): ##STR2## wherein A represents 5- or 6-membered heterocyclic ring having 1-3 nitrogen atoms, the ring being optionally substituted by at least a member selected from the group consisting of C₁₋₅ alkyl, cyano, hydroxyl, C₁₋₅ alkoxyl, amino, C₁₋₅ alkylamino, C₂₋₆ dialkylamino, C₂₋₅ acylamino, carboxyl, C₂₋₅ alkoxycarbonyl and carbamoyl, and R¹ and R² independently represent hydrogen atom or C₁₋₅ alkyl or R¹ and R² may form together C₁₋₅ alkylene, and salts thereof.

DETAILED DESCRIPTION OF THE INVENTION

The group A in the general formula (I) includes, for instance, pyridyl, pyridazinyl, pyrimidinyl, pyrazinyl, sym-triazinyl, asym-triazinyl, pyrrolyl, imidazolyl, pyrazolyl and the like, which may also be substituted by at least one substituent. By way of illustrating the substituent, mention may be made of normal or branched C₁₋₅ alkyl such as methyl, ethyl, propyl, butyl and pentyl; cyano; hydroxyl; normal or branched C₁₋₅ alkoxyl such as methoxy, ethoxy, propoxy and butoxy; amino; normal or branched C₁₋₅ alkylamino such as methylamino, ethylamino, propylamino and butylamino; normal or branched C₂₋₆ dialkylamino such as dimethylamino and diethylamino; normal or branched C₂₋₅ acylamino such as acetylamino, propionylamino and butyrylamino; carboxyl; normal or branched C₂₋₅ alkoxycarbonyl such as methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl and butoxycarbonyl; and carbamoyl.

Examples of pyridazinone derivatives having the general formula (I) are the following compounds: ##STR3##

Pharmaceutically acceptable salts of pyridazinone derivatives having the general formula (I) are also included in the scope of the present invention. By illustrating those salts, mention may be made of salts of inorganic acid such as hydrochloric acid and phosphoric acid and of organic acid such as lactic acid and acetic acid. All these compounds are useful as cardiac stimulants.

Preparation of the compound according to the present invention will be described below.

A pyridazinone derivative according to the present invention, for example, may be prepared as follows: ##STR4## wherein A, R¹ and R² are as defined hereinabove and X represents a halogen atom.

That is, a desired pyridazinone derivative (I) may be synthesized by heating a mixture of a compound (II) and a compound (III) in a polar solvent such as dimethylformamide, dimethylacetamide and phenol at 50°-200° C. for about 0.5-10 hours. A copper compound may be used as a catalyst.

A compound (II) is a known compound described in Journal of Medicinal Chemistry, 17, 273-280 (1974).

The compounds according to the present invention, when used as a cardiac stimulant, may be administered suitably by an oral and parenteral route. By way of illustrating available dosage unit form, mention may be made of powder, granule, tablet, sugar-coated tablet, pill, capsule, solution and the like in case of oral administration, and suppository, suspension, solution, emulsion, ampoule, injectable solution and the like in case of parenteral administration. A combination of these dosage forms may of course be also available. These dosage forms of the compound according to the present invention may be formulated pharmaceutically by the usual way in the art. Dosage can be determined by a physician according to age, sex, body weight, sensitivity to the drug, degree of symptom and physical condition of a patient, administration route, duration and interval of administration, properties, formulation and type of pharmaceutical preparation, kind of active ingredient and so on. Dosage range, for example, is 0.01-30 mg/kg/day, and preferably 0.05-10 mg/kg/day by oral administration, but is not restricted by the above.

The invention will be more clearly understood with reference to the following examples, but these examples are not to be construed to limit the scope of the invention.

EXAMPLE I Preparation of 6-[4-(4'-pyridyl)aminophenyl]-3(2H)-pyridazinone ##STR5##

A mixture of 2.33 g of 4-bromopyridine hydrochloride and 1.87 g of 6-(4-aminophenyl)-3(2H)-pyridazinone was reacted in 30 ml of N,N-dimethylformamide under the nitrogen gas flow at 110° C. for 4 hours.

After cooling the reaction mixture, 75 ml of water was added. The reaction mixture was neutralized with 2N sodium hydroxide followed by the filtration of deposited crystal.

The crude crystal thus obtained was recrystallized from a mixed solvent of ethanol-water and thoroughly purified with a silica gel chromatography to remove a trace amount of impurity contained in the crystal.

Fractions containing a desired substance were gathered, and the solvent was evaporated. Addition of HCl-ethanol and of ether to the condensate dissolved in a small amount of methanol yielded 0.23 g of 6-[4-(4'-pyridyl)aminophenyl]-3(2H)-pyridazinone hydrochloride.

    IR(KBr): 1650 cm.sup.-1

EXAMPLE II

Pharmacological and toxicological studies of a pyridazinone derivative according to the present invention were carried out by the following methods to show their utility as a cardiac stimulant.

1. Effect on contraction of an isolated and cross-circulated papillary muscle preparation of the dog

An isolated and cross-circulated papillary muscle preparation of the dog was prepared by Endo and Hashimoto's method (referred to American Journal of Physiology, 218, 1459-1463, 1970). The effect of the compound was measured by closely intraarterially injecting the compound dissolved in a solvent to the papillary muscle for the purpose of recording its effect on contraction of papillary muscle. Rate of increase in contraction of papillary muscle is shown in Table 1.

2. Effect on contraction of an isolated left atrium of the guinea pig

A left atrium was isolated from a male guinea pig with 200-300 g of body weight soon after striking on the back of its head. The mitral orifice was fixed to the bottom of an organ bath filled with 30 ml of Krebs-Henseleit solution maintained at 35° C. A gas mixture comprising 95% of O₂ and 5% of CO₂ was passed through the Krebs-Henseleit solution in the organ bath. Isometric tension was measured by connecting a left auricle of heart and a transducer by a yarn. A resting tension of 0.5 g was given to the atrium, which was then electrically driven with square pulses with duration of 1 msec and voltage of 1.5 times as much as threshold in the rate of 2 Hz via dipolar platinum electrodes.

After stabilizing the atrium for 30 minutes from its preparation, the compound dissolved in a solvent was added to the organ bath to measure its effect. The rate of increase in contraction of left atrium is shown in Table 1.

3. Effects on myocardial contraction in anesthetized dogs

Male and female mongrel dogs with body weight of 8-15 kg were used. A dog was anesthetized by intraveneous injection of 30 mg/kg of sodium pentobarbital and practiced artificial respiration. The dog was thoractomized between fourth and fifth costa which was cut off. The pericardium was incised to expose heart. Blood flow through the aorta, which was measured with an electromagnetic blood flowmeter whose probe was attached to the ascending aorta, was used as an approximate index of cardiac output (CO). Left ventricular pressure (LVP) was measured with a Miller Catheter-tip pressure transducer and the first derivative of the LVP (dP/dt) was measured with a differentiater. Contraction of right ventricular muscle (Cont) was determined with a strain-gauge attached to the wall. Systemic blood pressure was measured from the left femoral artery. Heart rate was measured with electrocardiogram (lead II) and a cardiotachometer. The compound dissolved in a solvent was administered intravenously from a left femoral vein.

Maximum value of dP/dt (dP/dt max) and rate of increase in Cont and CO are shown in Table 1.

4. Acute Toxicity

Acute toxicity (LD₅₀) for intravenous administration in male mice was determined by the method of Richfield and Wilcoxon (referred to Journal of Pharmacology and Experimental Therapeutics, 96, 99-113, 1949). The result is shown in Table 1.

                                      TABLE 1                                      __________________________________________________________________________                            Contraction of                                                                         Contraction of                                                         papillary                                                                              left atrium of                                                         muscle of dog                                                                          quinea pig                                                                             Anesthetized dog    LD.sub.50                                  dosage                                                                             in-     in- dosage                                                                             dP/dt max                                                                            Cont CO   (mg/                                       (μg                                                                             crease                                                                             dosage                                                                             crease                                                                             (μg/kg                                                                          increase                                                                             increase                                                                            increase                                                                            kg)                 Compound               i.a.)                                                                              (%) (g/ml)                                                                             (%) i.v.)                                                                              (%)   (%)  (%)  i.v.                __________________________________________________________________________      ##STR6##              30  12.4                                                                               10.sup.-5                                                                          163  30  64   31   13   103                 Example I                              100 108   39   11                       __________________________________________________________________________ 

What is claimed is:
 1. A pyridazinone compound having the formula (I): ##STR7## wherein A represents a member selected from the group consisting of pyridyl, pyridazinyl, pyrimidinyl, pyrazinyl, sym-triazinyl, asym-triazinyl, pyrrolyl, imidazolyl and pyrazolyl, with substituent A being optionally substituted by at least one member selected from the group consisting of C₁₋₅ alkyl, cyano, hydroxyl, C₁₋₅ alkoxyl, amino, C₁₋₅ alkylamino, C₂₋₆ dialkylamino, C₂₋₅ acylamino, carboxyl, C₂₋₅ alkoxycarbonyl and carbamoyl, and R¹ and R² independently represent hydrogen or C₁₋₅ alkyl or R¹ and R² together may form C₁₋₅ alkylene, or a salt thereof.
 2. A pharmaceutical composition useful as a cardiac stimulant, comprising:a therapeutically effective amount of the compound as set forth in claim 1 together with a pharmaceutically acceptable carrier.
 3. A method of treating cardiac insufficiency in humans, which comprises:administering thereto a therapeutically effective amount of the composition as set forth in claim
 2. 4. The pyridazinone compound as set forth in claim 1, wherein A is imidazolyl substituted by methyl and/or hydroxyl, or pyridyl substituted by at least one member selected from the group consisting of methyl, cyano, hydroxyl, methoxy, amino, dimethylamino and carbamoyl.
 5. The pyridazinone compound as set forth in claim 1, wherein both R¹ and R² represent hydrogen.
 6. The pyridazinone compound as set forth in claim 1, wherein R¹ or R² represents methyl.
 7. 6-[4-(4'-Pyridyl)aminophenyl]-3(2H)-pyridazinone.
 8. The method of claim 3, wherein said human is administered from 0.01-30 mg/kg/day of said pyridazinone compound.
 9. The method of claim 8, wherein the dosage range of administration of said pyridazinone compound ranges from 0.05-10 mg/kg/day. 